Once, while playing the DOS version of Bard's Tale 1, my hunter hit for a negative amount of damage.

Has anyone else had something like this happen? If so, did the attack kill the enemy?

(In my case, the attack was a critical hit, so I don't know if it would have killed the enemy anyway.)

I have never played the DOS version because it looks and sounds awful, and has so many bugs.

However, there are variety of known overflow bugs.

1 - Hunter skill overflows in the Apple II/ C64 version of BT1 so that at high levels, hunters stop critting.
2 - The calculation for hitting overflows in Bards' Tale 2 on Apple II / C64 / DOS in BT2. It's known that both BT1 & 2 use your armor class as a primary input for accuracy to hit, so it's quite possible that BT1 just doesn't feature low enough armor to expose the bug.
3 - I once received -2 gold, but I forget which platform and game.

A few observations about the DOS versions with respect to those things you have listed (note that I do not consider the DOS versions of 1 and 2 to be broken as I do with 3):

1. Hunter and Rogue skill both overflow in DOS BT2 but not DOS BT1. Of course, Rogues seem to be completely broken in BT1 and BT2, so you can't know this without examining the characters with a hex viewer/editor or transferring to BT3. (Then again, even in versions where they work, do they have any actual use?) (Fortunately, even in the broken DOS BT3, Rogue skills *do* work correctly, as does the Hunter's critical hit.)
2. In BT2 DOS, I haven't had any trouble hitting enemies with L+ AC. On the other hand, for a large portion of the game, I have noticed that enemies are unable to hit my characters. Also, enemy spells seem to always fail to hit anyone with double digit levels. (I have been up against enemies that spam Mangar's Mallet and not been hit once.) However, if the monster joins my party and becomes hostile, then the monster can suddenly hit my party with attacks and spells.

By the way, in the fight where my Hunter did negative damage, there were spellcasting enemies, and I suspect one of the enemies kept using a spell that lowers the damage inflicted by the party's attacks.

Yes, the to-hit calculations are pretty asymmetric. in all versions (of BT1 & BT2), you're very rarely hit with max AC, and nearly always will hit the opponents. However, in Apple IIgs, C64, and Amiga BT2, you can be hit with L+ AC, it is just quite uncommon.

I'm quite doubtful that L+ is actually the in-memory maximum as well. I'm not sure how it plays out across versions, but it seems like testing I've done with differently levelled monks suggests their chance to not be hit improves after L+. More concretely, giving my characters arbitrarily high dexterity on the Amiga version, where stats like 99 dexterity work properly and don't overflow made me pretty much unhittable.

Note that the Hunter skill overflow I refer to is the live calculation of hit chance, which relates to stored values but is not limited to it.